Ooh, I struggled with this question before we went on our trip earlier this month. Here's what I found after taking TONS of pictures with different speeds of film and under different conditions:
800 is best for nighttime and parades. I just left my camera set on auto flash, so *it* could "choose" to use flash or not. Lots of my photos turned out beautifully, in great color. The ones that appeared grainy or fuzzy were taken when I was using my zoom (since by zooming you are reducing the amount of available light). So, after getting back a couple batches of zoom-fuzzy film, I made sure not to zoom at night. My photos turned out really well after that.